The thermosetting resins that are commonly used today in fiber-reinforced composites cannot be reshaped after thermoforming. Because errors in forming cannot be corrected, these thermosetting resins are undesirable in many applications. In addition, these thermosetting resins, made from relatively low molecular weight monomers, are relatively low molecular weight, and often form brittle composites that have relatively low thermal stabilities.
Although thermoplastic resins are well known, practical aerospace application of high performance, fiber-reinforced thermoplastic resins is relatively new. Fiber in such composites toughens and stiffens the thermoplastic resin. While the industry is exploring lower temperature thermoplastic systems, like fiber-reinforced polyolefins or PEEKs, our focus is on high performance thermoplastics suitable, for example, for primary structure in advanced high speed aircraft including the High Speed Civil Transport (HSCT). These materials should have high tensile strength, and high glass transitions. Such materials are classified as "engineering thermoplastics." At moderate or high temperatures, the low performance, fiber-reinforced thermoplastic composites (polyolefins or PEEKS, for example) lose their ability to carry loads because the resin softens. Thus, improved thermal stability is needed for advanced composites to find applications in many aerospace situations. The oligomers of the present invention produce advanced composites.
Advanced composites should be thermoplastic, solvent resistant, tough, impact resistant, easy to process, and strong. Oligomers and composites that have high thermomechanical stability and thermo-oxidative stability are particularly desirable.
While epoxy-based composites like those described in U.S. Pat. No. 5,254,605 are suitable for many applications, their brittle nature and susceptibility to degradation often force significant design concessions when these epoxies are selected for aerospace applications. The epoxies are inadequate for applications which require thermally stable, tough composites, especially when the composites are expected to survive for a long time in a hot, oxidizing environment. Recent research has focused on polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness for these high performance applications. Still the maximum temperatures for use of the polyimide composites, such as those formed from PMR-15, can only be used at temperatures below about 600-625.degree. F. (315-330.degree. C.), since they have glass transition temperatures of about 690.degree. F. (365.degree. C.). PMR composites are usable in long term service (50,000 hours) at about 350.degree. F. (170.degree. C.). They can withstand temperatures up to about 600.degree. F. (315.degree. C.) for up to about five hundred hours.
PMR-15 prepregs, however, suffer significant processing limitations that hinder their adoption because the prepreg has a mixture of the unreacted monomer reactants on the fiber-reinforcing fabric, making them sensitive to changes in temperature, moisture, and other storage conditions, which cause the prepregs to be at different stages of cure. The resulting composites have varying, often unpredictable properties. Aging these PMR prepregs even in controlled environments can lead to problems. The reactants on the prepreg are slowed in their reaction by keeping them cold, but the quality of the prepreg depends on its absolute age and on its prior storage and handling history. And, the quality of the composite is directly proportional to the quality of the prepregs. In addition, the PMR monomers are toxic or hazardous (especially MDA), presenting health and safety concerns for the workforce. Achieving complete formation of stable imide rings in the PMR composites remains an issue. These and other problems plague PMR-15 composites.
The commercial long chain polyimides also present significant processing problems. AVIMID-N and AVIMID-KIII (trademarks of E. I. duPont de Nemours) resins and prepregs differ from PMRs because they do not include aliphatic chain terminators which the PMRs use to control molecular weight and to retain solubility of the PMR intermediates during consolidation and cure. Lacking the chain terminators, the AVIMIDs can chain extend to appreciable molecular weights. To achieve these molecular weights, however, the AVIMIDs (and their LARC cousins) rely on the melting of crystalline powders to retain solubility or, at least, to permit processing. It has proven difficult to use the AVIMIDs in aerospace parts both because of their crystalline melt intermediate stage and the PMR plague that these AVIMID prepregs also suffer aging.
So, research continues and is now turning toward soluble systems like those we described in our earlier patents, including acetylenic-terminated AVIMID-KIII prepregs of the Hergenrother (NASA-Langley) type. For these soluble systems, many different polyimide sulfone compounds have been synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode (U.S. Pat. No. 3,839,287) synthesized monofunctional, maleic-capped linear polyarylimides. Holub and Evans (U.S. Pat. No. 3,729,446) synthesized similar maleic or nadic-capped, imido-substituted polyester compositions.
For imides and many other resin backbones we have shown surprisingly high glass transition temperatures, reasonable processing parameters and constraints for the prepregs, and desirable physical properties for the composites by using soluble oligomers having difunctional caps, especially those with nadic caps. Linear oligomers of this type include two crosslinking functionalities at each end of the resin chain to promote crosslinking upon curing. Linear oligomers are "monofunctional" when they have one crosslinking functionality at each end. The preferred oligomers from our earlier research were "difunctional," because they had two functional groups at each end. Upon curing, the crosslinking functionalities provide sites for chain extension. Because the crosslinks were generally the weakest portions of the resulting composite, we improved thermo-oxidative stability of the composites by including two crosslinks at each junction. We built in redundancy, then, at each weak point. We maintained solubility of the reactants and resins using, primarily, phenoxyphenyl sulfone chemistries. Our work during the past fifteen years across a broad range of resin types or chemical families is described in the following, forty-nine United States Patents (all of which we incorporate by reference):
______________________________________ INVENTOR PATENT TITLE ISSUE DATE ______________________________________ Lubowitz et al. 4,414,269 Solvent Resistant November 8, 1983 Polysulfone and Poly- ethersulfone Com- posites Lubowitz et al. 4,476,184 Thermally Stable October 9, 1984 Polysulfone Composi- tions for Composite Structures Lubowitz et al. 4,536,559 Thermally Stable August 20, 1985 Polyimide Polysulfone Compositions for Composite Structures Lubowitz et al. 4,547,553 Polybutadiene October 15, 1985 Modified Polyester Compositions Lubowitz et al. 4,584,364 Phenolic-Capped April 22, 1986 Imide Sulfone Resins Lubowitz et al. 4,661,604 Monofunctional Cross- April 28, 1987 linking Imidophenols Lubowitz et al. 4,684,714 Method for Making August 4, 1987 Polyimide Oligomers Lubowitz et al. 4,739,030 Difunctional End-Cap April 19, 1988 Monomers Lubowitz et al. 4,847,333 Blended Polyamide July 11, 1989 Oligomers Lubowitz et al. 4,851,495 Polyetherimide July 25, 1989 Oligomers Lubowitz et al. 4,851,501 Polyethersulfone July 25, 1989 Prepregs, Composites, and Blends Lubowitz et al. 4,868,270 Heterocycle Sulfone September 19, 1989 Oligomers and Blends Lubowitz et al. 4,871,475 Method for Making October 3, 1989 Polysulfone and Polyethersulfone Oligomers Lubowitz et al. 4,876,328 Polyamide Oligomers October 24, 1989 Lubowitz et al. 4,935,523 Crosslinking June 19, 1990 Imidophenylamines Lubowitz et al. 4,958,031 Crosslinking September 18, 1990 Nitromonomers Lubowitz et al. 4,965,336 High Performance October 23, 1990 Heterocycle Oligomers and Blends Lubowitz et al. 4,980,481 Pyrimidine-Based December 25, 1990 End-Cap Monomers and Oligomers Lubowitz et al. 4,981,922 Blended Etherimide January 1, 1991 Oligomers Lubowitz et al. 4,985,568 Method of Making January 15, 1991 Crosslinking Imidophenyl-amines Lubowitz et al. 4,990,624 Intermediate February 5, 1991 Anhydrides Useful for Synthesizing Etherimides Lubowitz et al. 5,011,905 Polyimide Oligomers April 30, 1991 and Blends Lubowitz et al. 5,066,541 Multidimensional November 19, 1991 Heterocycle Sulfone Oligomers Lubowitz et al. 5,071,941 Multidimensional December 10, 1991 Ether Sulfone Oligomers Lubowitz et al. 5,175,233 Multidimensional December 29, 1992 Ester or Ether Oligomers with Pyrimidinyl End Caps Lubowitz et al. 5,082,905 Blended Heterocycles January 21, 1992 Lubowitz et al. 5,087,701 Phthalimide Acid February 11, 1992 Halides Lubowitz et al. 5,104,967 Amideimide Oli- April 14, 1992 gomers and Blends Lubowitz et al. 5,109,105 Polyamides April 28, 1992 Lubowitz et al. 5,112,939 Oligomers Having May 12, 1992 Pyrimidinyl End Caps Lubowitz et al. 5,115,087 Coreactive Imido May 19, 1992 Oligomer Blends Lubowitz et al. 5,116,935 High Performance May 26, 1992 Modified Cyanate Oligomers and Blends Lubowitz et al. 5,120,819 High Performance June 9, 1992 Heterocycles Lubowitz et al. 5,126,410 Advanced Heterocycle June 30, 1992 Oligomers Lubowitz et al. 5,144,000 Method for Forming September 1, 1992 Crosslinking Oligomers Lubowitz et al. 5,151,487 Method of Preparing a September 29, 1992 Crosslinking Oligomer Lubowitz et al. 5,155,206 Amideimide Oli- October 13, 1992 gomers, Blends and Sizings for Carbon Fiber Composites Lubowitz et al. 5,159,055 Coreactive Oligomer October 27, 1992 Blends Lubowitz et al. 5,175,234 Lightly-Crosslinked December 29, 1992 Polyimides Lubowitz et al. 5,175,304 Halo- or Nitro- December 29, 1992 Intermediates Useful for Synthesizing Etherimides Lubowitz et al. 5,198,526 Heterocycle Oligomers March 30, 1993 with Multidimensional Morphology Lubowitz et al. 5,210,213 Multidimensional May 11, 1993 Crosslinkable Oligomers Lubowitz et al. 5,216,117 Amideimide Blends June 1, 1993 Lubowitz et al. 5,227,461 Extended Difunctional July 13, 1993 End-Cap Monomers Lubowitz et al. 5,239,046 Amideimide Sizing August 24, 1993 For Carbon Fiber Lubowitz et al. 5,268,519 Lightly Crosslinked December 7, 1993 Etherimide Oligomers Lubowitz et al. 5,286,811 Blended Polyimide February 15, 1994 Oligomers and Method of Curing Polyimides Lubowitz et al. 5,344,894 Polyimide Oligomers September 6, 1994 and Blends ______________________________________
The heterocycles (i.e., oxazoles, thiazoles, or imidazoles) use a processing principle more akin to the AVIMIDs than the phenoxyphenyl sulfone solubility principle of our other resins. The heterocycles have poor solubility, even with our "sulfone" chemistries, but they at least form liquid crystals or soluble crystals in strong acids. To produce non-crystalline (amorphous) composites, we capitalize on the ability of our heterocycles to melt at the same temperature range as the cure and promote crosslinking in the melt. With relatively low molecular weight, capped, heterocycle oligomers, we can autoclave process these materials. Autoclave processing is a significant achievement for these heterocycles which present to the industry, perhaps, the most challenging problems. The polybenzoxazoles we produced, in addition, are useful at temperatures up to about 750-775.degree. F. (400-413.degree. C.), since they have glass transition temperatures of about 840.degree. F. (450.degree. C.). We describe multifunctional heterocycle and heterocycle sulfones in copending U.S. patent application Ser. No. 08/327,180, which we incorporate by reference.
Some aerospace applications need composites which have even higher use temperatures than these polybenzoxazoles while maintaining toughness, solvent resistance, ease of processing, formability, strength, and impact resistance. Southcott et al. discuss the problems of the prior art imide systems and the advantages of our soluble monofunctional and difunctional nadic-capped imide systems in the article: Southcott et al., "The development of processable, fully imidized, polyimides for high-temperature applications," 6 High Perform. Polym., 1-12 (U.K. 1994). For these extremely demanding requirements, our multidimensional oligomers (i.e., oligomers that have three or more arms extending from a central organic hub to yield three-dimensional morphology) have superior processing parameters over more conventional, linear oligomers that might produce composites having these high thermal stabilities. Our multidimensional oligomers can satisfy the thermal stability requirements and can be processed at significantly lower temperatures. Upon curing the end caps, the multidimensional oligomers crosslink so that the thermal resistance of the resulting composite is markedly increased with only a minor loss of stiffness, matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties. We can achieve glass transition temperatures above 950.degree. F. (510.degree. C.) with composites cured from our difunctional multidimensional oligomers (which we call "star-burst" oligomers). Of course, a full range of use temperatures are possible by selecting the hubs (which usually is an aromatic moiety), the backbone monomers used in the arms, end caps, and number of crosslinking functionalities per cap.
We now believe we can achieve even better properties in advanced composites by including an even higher number of crosslinking functionalities than the mono- or difunctional systems of the linear or multidimensional resins discussed in our earlier patents or patent applications. The higher density of crosslinks provide redundancy at those locations in the macromolecular, cured composite which are most susceptible to thermal degradation.